How to install FreeBSD on a ZFS system 'root' partition

Problem

ZFS — the breakthrough file system in FreeBSD 7 (ported from
Sun's Solaris 10 Operating System) delivers virtually unlimited
capacity, provable data integrity, and near-zero administration. However,
FreeBSD's sysinstall(8) does not yet support installing the system
onto anything more exotic than a commonly used UFS partition scheme.
Furthermore, FreeBSD's boot loader(8) cannot yet load the kernel and
modules from ZFS.

Solution

Install FreeBSD onto a bootable USB flash drive along with a copy of the
zfsboot script on it. Boot from the USB flash drive and run the
zfsboot script. This usbinstall script can be used to prepare a
bootable USB drive.

Implementation

The zfsboot script presented here will create the regular bootable 'a'
partition, but configure it as RAID-1 using gmirror(8). This approach
provides the possibility to directly boot from any of the drives by
configuring the BIOS boot order. This works even if only one drive is used,
although gmirror(8) will not be doing any mirroring.
See Ralf S. Engelschall's <rse@FreeBSD.org> excellent
System Disk Mirroring HOWTO
and note the implementation here is using GEOM mirror on partitions of a
slice on a disk for the boot partition.

Since we're forced to use a regular 'a' boot partition there is some advntage
in using a regular 'b' partion for swap. Due to the way ZFS is currently
implemented there is a known issue (in FreeBSD and OpenSolaris) where ZFS
needs to allocate memory to send an I/O request. When there is no memory,
ZFS cannot allocate any and thus cannot swap a process out and free it.
Such low memory conditions can cause the machine to effectively freeze —
unable to swap out to disk right when it is most crucial to do so. See
http://Lists.FreeBSD.org/pipermail/freebsd-current/2007-September/076831.html

The script assumes the FreeBSD 8 disk1 ISO to be mounted on /dist on another
host and /root/.ssh/authorized_keys on that host should allow the target machine
booted via the usb drive to connect without requiring a password.